[racket] Web-cells, anonymous functions, and garbage collection

Ah yes. We agree. I presume that Zack is concerned with the normal
circumstance of running a racket program from the command line, but I
recognize that you could do lots of creative things that will behave
properly.

Jay

On Thursday, February 14, 2013, Robby Findler wrote:

> That is also what I meant by code (well, I was speaking generally about
> all bytecode).
>
> The point I'm making is that this is not a limitation of our runtime
> system, it is just because you are imagining there are still references to
> that bytecode.
>
> In the specific instance you mention below, the namespace holding onto
> that module could become garbage. This happens in DrRacket a lot, for
> example.
>
>
> On Thu, Feb 14, 2013 at 7:45 AM, Jay McCarthy <jay.mccarthy at gmail.com>wrote:
>
> When I say "code", I mean module top-level binding byte-code. There's
> no way for Racket to remove
>
> (define (f x) (lambda (y) (printf "Gigantic string: ~a" (+ x y))))
>
> at the top-level. The byte-code will always contain the gigantic
> string. If you ever call f and get back something, that closure could
> be collected, but not the source that generated it.
>
> Jay
>
> On Thu, Feb 14, 2013 at 6:37 AM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
> > I'm pretty sure code is collected in (regular) Racket. Small numbers
> aren't
> > collected, true, but they also don't really take any space. Big numbers
> are
> > collected, too. (I'm not sure about interned symbols.)
> >
> > Robby
> >
> >
> > On Thu, Feb 14, 2013 at 7:26 AM, Jay McCarthy <jay.mccarthy at gmail.com>
> > wrote:
> >>
> >> In the Web server language, all anonymous functions are turned into
> >> top-level structures masquerading as functions where the fields are
> >> their free variables. For instance:
> >>
> >> (define (f x) (lambda (y) (+ x y))
> >>
> >> becomes, roughly,
> >>
> >> (struct a-lambda42 (x)
> >>  #:property prop:procedure
> >>  (lambda (the-structure y)
> >>   (define x (a-lambda42-x the-structure))
> >>   (+ x y)))
> >> (define (f x) (make-a-lambda42 x))
> >>
> >> In your program, the expression
> >>
> >> (web-cell-shadow wc-anonymous (λ (x) 'more-distinctive-text))
> >>
> >> becomes
> >>
> >> (struct a-lambda43 () #:property prop:procedure (lambda (s x)
> >> 'more-distinctive-text))
> >> (web-cell-shadow wc-anonymous (make-a-lambda43))
> >>
> >> Notice that the anonymous function has no fields because there are no
> >> free variables. The distinctive text is not shown anywhere in the URL
> >> because it is part of the code, not part of the data, of the closure.
> >>
> >> If you change the program slightly to
> >>
> >> (web-cell-shadow wc-anonymous
> >>                  (let () (define y 'more-distinctive-text)
> >>                       (λ (x) y)))
> >>
> >> then y is a free variable, so when you look at the URL:
> >>
> >>
> >>
> http://localhost:8000/;((%22c%22%20.%20%220((3)%204%20(((lib%20%5C%22web-server%2Flang%2Fabort-resume.rkt%5C%22)%20.%20%5C%22lifted.6%5C%22)%20((lib%20%5C%22web-server%2Flang%2Fweb-cells.rkt%5C%22)%20.%20deserialize-info:frame-v0)%20(%23%5C%22%2Ftmp%2Ft.rkt%5C%22%20.%20%5C%22lifted.1683%5C%22)%20(2%20.%20%5C%22lifted.1991%5C%22))%200%20()%20()%20(0%20(1%20(h%20-%20()%20(lifted.1485-0%20.%20this-is-a-song-about-Japan)%20(lifted.1521-1%202%20(b!%20.%20more-distinctive-text))))%20(c%20(v!%20(3)%20%23f%20%23f))))%22))
> >>
> >> You see the distinctive text.
> >>
> >> As far as GC goes, code is not generally collected in Racket anyways.
> >> Any data that is an intrinsic part of the program text (such as
> >> literal symbols and numbers) will always be around in your program
> >> whether you use #lang web-server or not.
> >>
> >> Jay
> >>
> >>
> >> On Wed, Feb 13, 2013 at 8:34 PM, Galler <lzgaller at optonline.net> wrote:
> >> > Good evening,
> >> >
> >> > I've been investigating lang web-server, performance and garbage
> >> > collection
> >> >
> >> > I have a question
>
>

-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93
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